In a cellular radio system, radio signals from multiple antennas of the cellular radio system can be coordinated to amplify the radio transmission in a desired direction, i.e. typically towards specific receivers, such as mobile phones, user equipments or the like. The coordination of the radio signals is achieved by applying different phase shifts to electrical signals fed to each of the multiple antennas, each of which generate a respective radio signal. In this manner, a resulting radio signal, i.e. the radio transmission, is generated by constructive interference between the respective radio signals.
The phase shift is given by a so called precoder, or precoder index. In a known example, a base station of the cellular radio system can rely on that the mobile phones report a preferred precoder. Each mobile phone can estimate the preferred precoder by measuring on pilot signals from the base station. The pilot signals, or reference signals, are known to the mobile phones. This means for example that each transmit antenna sends a known pilot signal in a known time/frequency slot, in which the other antennas are silent. The mobile phone thereby is able to estimate a channel response for each transmit/receive antenna pair, i.e. all combinations of transmit/receive antenna pairs, between the base station and the mobile phone. Each transmit/receive antenna pair corresponds to a certain precoder. Accordingly, the mobile phone may calculate its preferred precoder.
Information about the preferred precoder can also be used by the base station to estimate the direction to the receiver. If the reported preferred precoder changes with time, the base station can also detect movements of the mobile phone, which reported the preferred precoders.
In order to keep the number of reports of preferred precoders low, a selection of precoder choices is often limited. In this manner, a change in preferred precoder becomes more rare. A consequence thereof is that it is difficult to accurately estimate a movement of a single mobile phone.
In an exemplifying scenario, a group of users, each user carrying a respective mobile phone, are disembarked from a train and then the users head towards a parking lot. In this scenario, the use of precoders may enable improved performance if movement of the group relatively a base station serving the mobile phones of the users, a.k.a. group movement, may be detected. The precoders would then be set to follow the movement of the group. Other similar scenarios, include groups entering/leaving an event at a stadium, a concert in a concert hall or the like.
A problem in this regards may be how to efficiently detect group movement.